248 IEEE TRANSACTIONS ON ELECTRON DEVICES May

Electron and Hole Mobilities in Inversion Layers on Thermally Oxidized Silicon Surfaces -

0. LEISTIKO, JR., MEMBER, IEEE, A. S. GROVE, ASSOCIATE, IEEE, AND C. T. SAH, MEMBER, IEEE

Absfracf-Extensive measurements of electron and hole mo- bilities in inversion layers on thermally oxidized silicon surfaces were performed using the field effect conductance technique. It was found that both electron and hole mobilities are practically constant and approximately equal to one half of their respective bulk values up to a surface field of about 1.5 X 105 volts/cm, corresponding to about 10l2 electronic charges/cmz induced in the silicon. At higher fields the inversion layer mobilities begin to decrease slightly.

The temperature dependence of inversion layer mobilities follows a T-1.5 rule at the upper range of the interval -196 to 200°C, indicating a scattering mechanism similar to lattice scattering. This observation is further supported by the lack of a significant effect of an order-of-magnitude variation in the bulk impurity concentration - 1OI6 ~ m - ~ ) on the inversion layer mobilities.

No significant effect of structural and geometrical parameters (such as channel length and shape, oxide type and thickness, and surface charge density) was found on the inversion layer mobilities.

I. INTRODUCTION

HE FIELD EFFECT conductance technique has been extensively used in the past in the study of semiconductor surfaces. In this technique the change

in the conductance along a semiconductor surface is studied as a function of the electric field applied normal to the surface. The experimentally observed conductance is invariably smaller than what might be expected on theoretical grounds. The deviation has been attributed to two factors [I] : first, some of the excess charge brought to the surface by the applied electric field may be immobil- ized in surface states; and second, the mobility of carriers near a surface should be smaller than in the bulk due to additional scattering mechanisms associated with the pres- ence of the surface.

The two effects are difficult to separate on the basis of field effect conductance experiments alone. Coovert [2a] Albers [2b], and more recently, Fowler, Fang, and Hoch- berg [2c] modified the classical field effect experiment by also adding a normal magnetic field and performing, essentially, a surface Hall effect measurement. Many, et al. [3] has studied majority carrier surface mobilities in germanium samples using a pulsed field effect technique to eliminate surface state effects. An alternate approach became possible through the use of the MOS, or field effect capacitance technique [4] which can yield quantita-

Manuscript received November 18, 1964; revised February 3, 1965.

0. Leistiko, Jr. and A. S. Grove are with Fairchild Semiconductor, a Division of Fairchild Camera and Instrument Corp., Palo Alto, Calif.

C. T. Sah is with the University of Illinois, Urbana, Ill.

t,ive information about the nature and number of surface states or charges and thus, coupled with field effect con- ductance experiments, can lead to the separation of surface state and mobility effects.

A systematic study of thermally oxidized silicon surfaces was recently performed in this laboratory [5 ] . It was shown through the use of the MOS capacitance technique that a thermally oxidized silicon surface can be characterized by a surface charge, pictured to be located near the oxide- silicon interface, whose density per unit area &../a was found to be practically independent of the silicon surface potential. This finding was verified in literally thousands of measurements using both 2'- and N-type silicon of various doping levels which were oxidized under a variety of conditions. The density of these surface charges was found to range upward from 1 - 3 X 10" cm-' depending on the processing conditions, but was reproducible for a given set of conditions to within about 5 X 10" cm-' and stable even under prolonged beat treatment experi- ments performed under a bias of either polarity.

With this overall picture of the oxide-silicon interface now available, an experimental investigation of the field effect conductance of thermally oxidized silicon MOS structures was undertaken with the aim of determining the mobiIities of electrons and hoIes in surface inversion layers.

The principles of the measurement method are first reviewed, and then the experimental technique and the characteristics of the NIOS structures used in this in- vestigation are described. Finally, experimental results obtained on a wide variety of devices are presented. Both electron and hole mobilities in surface inversion layers were measured a t temperatures ranging between - 196°C and 20O"C, and with the resistivity of the semiconductor bulk ranging between 1 and 10 ohm-cm. In addition, the devices had various oxide thicknesses and geometries, and were characterized by surface charge densities which varied over a factor of five.

11. EXPERIMENTAL METHOD

A. Principles of the Measurement Method The conductance of an inversion layer formed on the

surface of an N-type semiconductor is given by

g = li "(x) dx (1)

where Z is the width of the device L the length of the inverted region, and x is the distance from the semi-

1965 Leistiko, Jr., et al.: Mobilities in Inversion Layers on Oxidized Silicon

conductor surface (Fig. 1). The point x = xi denotes the distance below the surface where the semiconductor is just intrinsic. The conductivity of the inversion layer a t a distance x below the surface is

dx) = qPP(x)Pb). (2)

Thus,

where

Q, = 4 lz' P(X> dx, (4)

and the efective value of the mobility perf is defined as

The charge per unit area Q, due to holes within the inversion layer can be calculated as a function of the total charge per unit area induced in the senziconductor Q,. The results of such calculations for silicon a t room temperature are given in Fig. 2 of [5]. The total charge induced in the silicon Q. can, in turn, be related to the voltage V , applied to the gate of the MOS structure shown in Fig. 1 of [5] by

4. - Q./Co = V , - 4Ls f Q s s / C o V' (6)

where C, E K O ~ O / x O is the capacitance per unit area of the oxide layer, Q,, is the surface charge density and +Ls is the metal-semiconductor work function difference. (For the case of an aluminunz field plate and the range of impurity concentrations encountered in this work +Ls is taken to be -0.7 volt for a P-type and zero for an N-type semiconductor [j].)

Thus, for a given MOS structure, and a t a given tem- perature, Q, can be calculated as a function of the effective voltage V' = V , - +Ls + Q.,/C,. Such a relationship is illustrated by the theoretical curve in Fig. 2 which corresponds to the case of an MOS device from Group 536, described in Table I. The experimentally measured con-

Device Group

(-.-

1010 E 1OlOB 1014D

52B 4B21

( 4B22

249 n

IN

Fig. 1. Illustration of the type of MOS structure used in the conductance experiments.

7 0

I P : - co

IC6 0.5

0 EXPERIMENT I

-7 -i2 -10 -L -6 -4 -L lO v, (volts)

Fig. 2. The number of minority carriers (per unit area) within the inversion layer and the source-to-drain conductance of an MOS structure from Group 536 (see Table I). Also shown are capacitance-voltage characteristics of a corresponding MOS capacitor.

The theoretical curves are represented as functions of the effective gate voltage V' V G - + ' ~ s + Q.,/Co; the experimental measurements as functions of the gate voltage VG. Theory and experiment were brought into agreement by taking - + I M s +- were determined by comparison of g and Q,/q according to (7). Qs./Co = 2 volts, i.e., Qs. /q = 2.9 X 10" The values of self

TABLE I CHARACTERISTICS OF THE EXPERIMENTAL DEVICE STRUCTURES

Bulk Surface Charge Oxide Channel Resistivity Density (ohm-em)

Thickness Length

( Microns NIicrons cm2/v-sec and bulk type Geometry L /LBu lk X0 Q s s / q

Symbol - ~ - ~ _ _ _ _ _ _ _ _ _ _ _ _ ~ ~

Circular 1250

56 0.20 56 10.5 P Circular

1250 10.5 P

0 2.9 X 10" P 5 . 1 X 10"

0.15 13 405 0.15 8

1 . 0 N Rectangular

V 470

3 .8 X loll 10.0 N Rectangular

9 0.15 11

4 . 5 x 10" 0.19 65 475 12.0 N Rectangular

LT 4 .6 X loll 0.20 6.5 470 10.5 N Circular

A 470

2 . 2 x 10'1 0.62 65 10.5 N Circular

470 10.5 N Circular e b 6 . 0 X 10'l

9 .6 X 10" 0.17 5 1010 1 . 6 P Rectangular

e 5 .6 X 1OI1 0.17 5 920 1 . 0 P Rectangular

5 . 5 x 101' 0.15 12 920 1 . 0 P Rectangular

k 6 . 9 X 10" 0.26 56 1250 10.5 P Circular

A 3 .8 X 10" 0.26

m

_____ ~ _ _ _ _ _ _ _ _ _ _ _ _ -- --___

250 IEEE TRANSACTIONS ON ELECTRON DEVICES

ductance of this device is also shown in Fig. 2 as a function of the gate voltage V,. The approximately 2-volt displace- ment between the %” and the V , axes, which corresponds to Q,,/q = 2.9 X 10’l em-’, insures good agreement between both the measured and the theoretical conduct- ance curves and the capacitance-voltage characteristics of a corresponding MOS capacitor shown in the lower portion of Fig. 2. (The high-frequency characteristics were obtained with 100-kc/s measurement frequency, in dark- ness; the low-frequency characteristics with the same frequency but with light shining on the device; the deple- tion-type characteristics with rapid dc biasing and with the device cooled. For a discussion of the three kinds of characteristics, see 151.)

By comparing values of &,/a with those of g a t a given V’ (or V,), the effective mobility can be readily deter- mined from

L g P e i f = - - Q,’

Mobility values obtained in this manner are shown in Fig. 2. The same calculations and measurements are pre- sented on a Cartesian graph, Fig. 3. This graph illustrates an important feature of the MOS theory: for a strongly inverted surface (Le., large negative values of V , in this case), Q, is given to a good approximation by the straight- line relationship - (V , - V,) /C, where V T denotes the “threshold” voltage of the MOS device [6]. It follows that if the surface charge density Q,, is a constant, a straight-line conductance vs. gate voltage plot implies a constant mobility value. The straight-line portion of the conductance curve in Fig. 3 indeed corresponds to the region of apparently constant mobility (peif = 201 emz/ volt-see) in Fig. 2. Naturally, the same values of mobility are obtained whether one uses the semilogarithmic or the linear representation for its evaluation.

In the structures used in this study, contact to the inversion layer was made through diffused regions. In the experiments reported here no dc bias was applied between these diffused regions and the bulk.

When there is such a dc bias applied, such as in the customary operation of the MOS transistor, the previously mentioned conductance analysis does not apply directly. Approximate analyses of the conductance of MOS tran- sistors in the presence of applied junction biases can be found in [6] and [7]. All of these analyses reduce to the previous formulae for the case of zero junction bias.

B. Measurement Technique Most of the measurements were taken with a circuit

especially designed for measuring small-signal conduct- ance. The circuit makes use of an operational amplifier connected to differentiate the drain current with respect to drain voltage. The ac-signal voltage was kept to a value of 10-mV rms at a frequency of 1 kc/s. From the output of the operational amplifier the signal proportional to g was fed to a logarithmic converter and then t o an x-y

t \ \ I

lo-** -16 -14 -12 -10 v, (vo l ts ) -8 -6 -4

May

Fig. 3. Cartesian plot of the curves shown in Fig. 2.

plotter. With a voltage ramp driving the gate of the device and the x axis of the plotter and the calibrated output of the logarithmic converter applied to the y axis, the graph of log g vs. V G could be quickly plotted in the form illustrated in Fig. 2.

In order to check the accuracy of this method, measure- ments were also taken in two other ways for comparison. In one, conductance was measured point-by-point with a standard conductance bridge. In the other, the drain current of the device I D was measured as a function of the drain voltage V , with V , as a parameter, and then the resulting I, vs. VD curves were graphically differen- tiated near the origin to obtain g. Results obtained by these three methods always agreed within 5 per cent.

Conductance measurements a t temperatures above 27°C were taken with the devices in a temperature monitored oven, and a t temperatures below 27°C with the devices immersed in liquids of known boiling points.

C. Experimental IC1 OX Transistor Xtructures A large number of MOX structures were fabricated for

use during the course of the present experiments. These structures can be broken into groups by characterizing them with certain parameters as is done in Table I. Aside from the variations indicated in this table, the devices were made in essentially the same way as described here.

Wafers of silicon cut parallel to the (111) plane of Czochralski-grown crystals were first chemically polished and then thermally oxidized either in dry oxygen a t atmospheric pressure and at 12OO0C, or in oxygen bubbled through 97°C water, also at 1200°C. Some runs of devices were given a post-oxidation heat treatment in 1050°C

1966 Leistiko, Jr., et al.: Mobilities in Inversion Layers on Oxidized Silicon 25 I

argon bubbled through 27°C water.' Photolithographic techniques were then used to delineate the desired patterns and standard diffusion techniques were employed to form the contact islands (Fig. 1).

Both rectangular and circular geometries were used (for the circular geometry see [6]). Channel lengths were varied between 5 and 70 microns, bulk resistivities between 1 and 10 ohm-cm (both P- and N-type), and oxide thick- ness between 0.15 and 0.62 micron.

Considerable care was exercised to insure the repro- ducibility of device parameters resulting from a given process and to insure the stability of the resulting struc- tures. Devices used in the detailed conductance measure- ments were chosen from a group of typically 20 similar units, so that their overall electrical characteristics were representative of the average of their group. As an indica- tion of the levels of stability attained, a group of 30 devices of various kinds were lifetested in bias conditions involving both positive and negative gate biases of 15 volts for over 3600 hours a t 150°C. No more than 10 per cent of the units showed any degradation of characteristics, and whatever degradation was observed was relatively minor.'

111. EXPERIMENTAL RESULTS

A. General Observations; Field Dependence of the Mobility The effective mobility of electrons and holes is shown

as a function of the total charge per unit area Q 8 / q induced in the semiconductor in Figs. 4 and 5, respectively.

It might be useful to point out that throughout this paper empty symbols denote hole mobilities while full symbols denote electron mobilities. Circular symbols of either kind correspond to samples of approximately 1 ohm-cm bulk resistivity, triangles to 10 ohm-em. Other variations in structural parameters are denoted by flags attached to the experimental points.

The general dependence of the mobility on the applied normal field or, equivalently, the total charge per unit area induced in the semiconductor, can be divided into two regions: in the first, up to approximately IQ,/ql = lo1' cm-', the mobility is practically constant. Beyond this point, the mobility decreases with increasing field. These results were confirmed by detailed and precise comparison of plots like those shown in Fig. 2 but with the voltage axis greatly expanded to increase the sensi- tivity of the comparison.

B. Effect of Structural and Surface Parameters on Mobility As the device parameters listed in Table I indicate,

the MOS transistor structures used in this study had different geometries, bulk doping concentrations, oxide thicknesses, channel lengths and surface charge densities,

on the surface charge density of oxidized silicon was recently 1 The effect of various degrees of hydration of thermal oxides

discussed by Kuper and Nicollian [SI. 2 A detailed study of the stability of MOS transistors fabricated

in a similar manner was recently presented [9].

Fig. 4. Inversion layer mobility of electrons as a function of the

meaning of the various symbols see Table I. total charge per unit area induced in the semiconductor. For the

Fig. 5. Inversion layer mobility of holes as a function of the total charge per unit area induced in the semiconductor. For the meaning of the various symbols see Table I.

I .o I I j

Fig. 6. The ratio of inversion layer to bulk mobilities of electrons

unit area induced in the semiconductor. For the meaning of the and holes as a function of the magnitude of the total charge per

various symbols and for the values of the bulk mobilities used in the reduction of the data, see Table I.

the latter owing to the different surface preparation processes employed. Although there is a considerable scatter noticeable in Figs. 4 and 5, it is believed to be due to uncertainties in the structural parameters used in the computation of the mobilities (such as channel length and oxide thickness), rather than due to the effect of these parameters on the carrier mobilities themselves.

It is particularly noteworthy to emphasize that this conclusion applies also to the bulk resistivity which was varied by an order of magnitude for both P-type and N-type bulk cases (1-10 ohm-cm). The lack of a strong dependence of the mobility on the impurity concentration would indicate that the controlling scattering mechanism is somewhat like lattice scattering. This observation was given further support by measurements of the effect of temperature on the mobility, discussed in the next section.

The effect of the redistribution of impurities during thermal oxidation [lo], which can cause the impurity concentration near the oxide-silicon interface to deviate from that in the bulk, was approximately taken into

252 IEEE TRANSACTIONS ON ELECTRON DEVICES M U Y

Fig. T (OK)

7. The effect of temperature on the inversion layer mobility of electrons ( [&./q 1 < 1012 em-2).

Fig. 8. The effect of temperahwe on t,he inversion layer mobility of holes ( j & , / q ] < 10lz em-2).

J

t Fig. 9. The relationship between inversion layer mobility and

the total charge per unit area induced in the semiconductor a t various temperatures.

account in a number of cases. It was found to have only a very minor effect on the mobility values.

All of the mobility data are summarized in Fig. 6 where the ratio of the inversion layer mobility to the corre- sponding value of the bulk mobility is plotted as a function of the magnitude of the total charge per unit area induced in the semiconductor. Again, all devices exhibit the same general tendency. It is evident that for 1&./q/ < 10" em-', the value of the mobility is approximately half of the corresponding bulk mobility value.

are shown in Fig. 9. It appears that the temperature dependence of peff is the same at any given value of / Q S / q / and that, conversely, the dependence of p e f i on jQS/qj is the same a t any given temperature in the range of the experiments.

It should be noted that for all of the devices investigated here the value of the surface charge density &,./a was essentially unaffected by the variation in temperature used in these experiments in agreement with results ob- tained by the capacitance-vol-tage method [ 5 ] .

C. E$ect of Temperature on the Mobility IV. DISCUSSION The dependence of the effective mobility on temperature

is shown in Figs. 7 and 8. The data given here correspond to the range where the mobility is approximately con- stant, i.e., I&./ql < lo1' cnC2. It appears that both electron and hole mobilities in inversion layers have a T-1.5 power dependence a t higher temperatures.

To indicate the effect of temperature on mobilities a t higher values of [ Q . / q / , results obtained for a typical device

Perhaps the most intriguing finding of this work is the apparent constancy of the inversion layer mobility a t relatively low surface fields (up to 1 2 3 . 1 = 1.5 X IO5 v/cm or i&,/ql = 10'' cm-'), which was observed in every MOS structure examined in the course of this work.

Schrieffer's [Ill theory for diffuse surface scattering predicts that the inversion layer mobility should decrease with increasing surface field due to the increasing influence

I965 Leistiko, Jr., et ai.: Mobilities in Inversion Layers on Oxidized Silicon 253

1.0

i- Pbulk

0.1 IOIO IO1'

Fig. 10. Comparison between typical experimental inversion layer mobilities and Schrieffer's constant field theory, using bulk mobilities in the reduction of the experimental data and also in the theoretical calculations.

of the surface on the carrier transport. In addition, it is generally assumed that, as the field near the surface is decreased, the mobility will approach the corresponding bulk mobility value. Instead, the inversion layer mobility approaches something like one half of the bulk value.

This experimental finding might lead to the conclusion that the mobility of carriers near a thermally oxidized silicon surface is about half of their bulk mobility even in the abscence of surface scattering effects per se. Since the silicon dioxide-silicon interface must be gradual, it is not a t all inconceivable that an imperfect transition region extends some distance into the silicon. If the inversion layer was always contained within such an imperfect tran- sition region, the mobility could only approach some value lower than the corresponding bulk mobility.

Figures 10 and 11 lend some support to this hypothesis. In these figures typical electron and hole mobilities are shown as a function of the total charge induced in the silicon. In Fig. 10, the nlobilities are normalized with the true bulk mobilities; in Fig. 11 they are normalized with the constant value they reach at IQJqI < 10" The solid lines in these figures represent Schrieffer's 1111 con- stant field theory for completely diffuse surface scattering, calculated using the true bulk mobility and the constant limiting mobility, respectively. Figure 11 appears to provide a more reasonable picture indicating an only partially diffuse surface scattering process, which is not inconsistent with the results obtained by Many, et al. [3] for germanium surfaces.

V. CONCLUSIONS

Extensive measurements of electron and hole mobilities in inversion layers on thermally oxidized silicon surfaces were performed.

Inversion layer mobilities of both electrons and holes appear to be constant up to surface fields of about 1.5 X lo5 volts/cm or ]Q,/pl = 10" em-'. This constant value is approximately equal to one half of the respective carrier bulk mobiIities. Beyond this point, the inversion layer mobilities decrease slightly with increasing surface fields.

The temperature dependence of inversion layer mobil- ities follows a T-l.' rule at the upper range of the interval - 196 to 200°C, indicating a scattering mechanism similar to lattice scattering.

Variation over an order of magnitude in the bulk

1.0, I SPECULAR,

i_ Pconst.

Fig. 11. Comparison between typical experimental inversion layer mobilities and Schrieffer's constant field theory, using the experi- mentally determined inversion layer mobilities at I&?/qI < data and also in the theoretical calculations. lo1* em) denoted by peonst, in the reduction of the experimental

impurity concentration has no significant effect on the inversion layer mobilities, in further agreement with a lattice scattering type mechanism.

KO significant effect of structural and geometrical pa- rameters (such as channel length and shape, oxide type and thickness, and surface charge density) on the inversion

mobilities was found.

VI. LIST OF SYMBOLS Capacitance of the MOS structure per unit Capacitance of the oxide layer 1 area. Source to drain conductance of the inversion layer. Drain current. Dielectric constant of the oxide. Length of the inverted region. Hole concentration Magnitude of the electronic charge. Charge per unit area due to holes. Charge per unit area induced in the semiconductor. Surface charge per unit area. Drain voltage. Voltage applied to the metal field plate relative to the semiconductor (gate voltage). Threshold voltage of the MOS structure (onset of inversion). Effective voltage applied to the MOS structure. Depth a t which the semiconductor is just intrinsic. Oxide layer thickness. Width of the inversion region. Permittivity of free space 8.859 x 10-l' farad/cm or 55.4 electronic charge /vp. Mobility in the bulk of the semiconductor. Constant value in effective mobility. Effective mobility of the inversion layer. Mobility of holes. Conductivity. Surface potential: Metal-semiconductor work function difference.

ACKNOWLEDGMENT The authors wish to thank C. A. Bittmann and V. G. K.

Reddi for helpful discussions and D. Hilbiber for designing the instrumentation used in this work.

254 IEEE TRANSACTIONS ON ELECTRON DEVICES LWUy

REFERENCES

[l] Brown, W. L., N-type surface conductivity on P-type ger- manium, Phys. Rev., vol 91, Aug 1953, pp 518-527.

[2a] Coovert, R. E., Surface magnetoconductivity experiments in silicon, J . Phys. Chmz. Solids, vol 21, 1961, pp 87-98.

[2b] Albers, W. A., Jr., Effective mobilities of surface carriers in germanium, J . Phys. Chem. SoZids, vol 23, 1962, pp 1249-1268.

[Zc] Fowler, A. B., F. Fang, and F. Hochbert, Hall measurements on silicon field effect transistor structures, IBM J . Res. De-

[3] Many, A., N. B. Grover, T. Goldstein, and E. Harnik, Surface veloprn., 1964, pp 427-429.

mobility measurements in germanium, J . Phys. Chem. Solids,

[4] For the earlier literature dealing with this method see the following.

Moll, J. L., Variable capacitance with large capacitance change, 1959 I R E Wescon Convention Record, pt 3, Aug 18-21,

Pfann, W. G., and C. G. B. Garrett, Semicondqctor varactors using surface space-charge layers, Proc. I R E (CorresponderLce),

design of surface space-charge varactors, Solid-State Electronics, Frank], D. R., Some effects on material parameters on the

Terman, L. M., An investigation of snrface states a t a silicon/silicon oxide interface employing metal-oxide-silicon diodes, Solid-State Electronics, vol 5, 1962, pp 285-299.

Linder, EL., Semiconductor surface varactor, Bell Sys. Tech.

Lehovec, K., A. Slobodskoy, and J. L. Spragne, Field effect

VOI 14, 1960, pp 186-192.

p 32-36.

V O ~ 47, 1959, pp 2011-2012.

v01 2: 1961, pp 71-76.

J., V O ~ 41, 1962, pp 803-831.

capacitance analysis of surface states on silicon, Phys. Stut .

[5] Grove, A. S., B. E. Deal, E. H. Snow, and C. T. Sah: Investiga,- tion of thermally oxidized silicon surfaces using metal-oxide- semiconductor structures, presented at the IEEE Solid-State Device Research Conf., Boulder, Colo., Jul 1964; Solid-State

[6] Sah, C. T., Characteristics of the metal-oxide-semiconductor Electronics, vol 8, 1965, pp 145-163.

transistors, I E E E T r a n s . o n Electron Devices, vol ED-11, 1964,

171 Borkan, H., and P. K. Weimer, An analysis of the characteristics of insulated gate thin-film transistors, RCA Rev., vol 24, 196:3,

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pp 153-165. Hofstein, S. R., and F. P. Heiman, The silicon insulated-gate

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field-effect transistors, Solid-State Electronics, vol 7, 1964, pp Ihantola, H. K. J., and J. L. Moll, Design theory of a surface

423430. Kuper, A. B., and E. H. Nicollian, Effect, of oxide hydration on surface potential of oxidized P-type silicon, presented a t the Electrochemical Society Meeting, Washington, U. C., Oct 1964 (abstract 131). Lamond, P., J. Kelley, and M. Papkoff, Reliability and failure modes in metal-oxide-silicon transistors, present,ed at the IEEE Electron Devices Meeting, Washington, D. C., Oct, 1964 (to appear in Electro-Technology). Grove, A. S., 0. Leistiko, Jr., and C. T. Sah, Redistribution of acceptor and donor impurities during thermal oxidation of silicon, J . A p p l . Phys., vol 35, 1964, pp 2695-2701. Schrieffer, J. R., Effective carrier mobility in surface-space charge layers, Phys. Rev., vol 97, 1955, pp 641-646.

A CornParison of Radiation Tolerance of Field Effect and Bipolar Transistors

B. L. GREGORY, MEMBER, IEEE, AND F. M. SMITS, SENIOR MEMBER, IEEE

Abstract-An analytical comparison of the radiation tolerance of conventional silicon field effect transistors and of silicon bipolar transistors has been performed. The channel or base thickness has been used as the respective critical variable, since it measures the degree of difficulty of device fabrication. For field effect transistors, the pinch-off voltage has been used as a free parameter. Based on recent lifetime degradation data for bipolar transistors and on carrier removal data for material of variable resistivity, it is shown that field effect transistors are not inherently more radiation tolerant than bipolar transistors. Only field effect transistors with pinch-off voltages well in excess of IO volts appear superior to bipolar tran- sistors.

INTRODUCTION HERE ARE two basic types of transistors presently in use, the common bipolar transistor and the uni- polar field effect transistor. In the bipolar transistor,

transport of minority carriers across the base region is controlled by the bias of the base region with respect

1964.

Corp., Albuquerque, N. Mex.

Manuscript received September 4, 1964; revised November 30,

The authors are with the Material and Device Div., Sandia

to the emitter. In the field effect transistor, the conduct- ance of a thin channel is controlled by the width of the space-charge region resulting from the reverse bias be- tween the channel and a “gate” electrode [I].

The characteristics of both types of devices are ad- versely affected by neutron irradiation. In the bipolar transistor, the reduction in minority carrier lifetime causes an increase in the number of minority carriers reconlbining within the base region, resulting in a reduction in current gain. In the field effect transistor, the property of the radiation-induced defects to act as deep traps for majority carriers causes a decrease in the conductivity of the channel, resulting in reductions in bias current and trans- conductance.

In material in the doping range normally employed for device construction, the reduction in minority carrier lifetime becomes significant a t flux levels well below those a t which the conductivity of the material is signif- icantly altered. For this reason it has been assumed in the past that the bipolar transistor would be more radia- tion sensitive than the field effect transistor. However,